Chromosomal deletions are indispensible tools for functional genomic analysis in the model organism Drosophila melanogaster. They are used by geneticists across the spectrum of research interests in both the mapping and isolation of the gene sequence responsible for a phenotype and in defining the cellular function of a gene identified from sequence data. Presently, 70 to 75 percent of the Drosophila euchromatic genome is represented in chromosomal deletions. The remaining 25 to 30 percent of the genome is distributed in 90 gaps between existing deletions. As a service to the research community, scientists at the Bloomington Drosophila Stock Center propose to generate chromosomal deletion strains that will fill these gaps and provide essentially complete coverage of the Drosophila genome. The project will consist of three steps: (1) the existence of a gap will be verified or deletion overlaps will be confirmed in cases of breakpoint uncertainty by genetic complementation studies, (2) deletions for each of the verified gaps will be generated by targeting each gap in turn with its own irradiation mutagenesis screen, and (3) each new chromosomal deletion will be characterized genetically and cytologically and placed in the Bloomington deficiency collection for worldwide distribution. The straightforward and efficient methodology for isolating new deletions will involve the irradiation-induced loss of single P transposable element insertions which have been precisely mapped by the Berkeley Drosophila Genome Project. Despite the biological limitations on the deletion of haploinsufficient loci, deletion coverage of the fly genome will probably reach 98 percent with this systematic effort. With the entire sequence of the Drosophila genome only a few years away, it is imperative that tools for functional genomics of the entire genome be readily available. Deletion coverage of the fly genome will increase the utility of Drosophila as a model organism and will strengthen the ability of basic research in flies to inform biomedical studies of human health and development.